Based on the internet novel of the same title and written by Guiyeoni, Do Re Mi Fa So La Si Do is a touching love story with bittersweet elements.

Jung-won wears a funny dragon suit, while working at the amusement park, and pours soda on Eun-gyu, because of his taunts. A short while later, Jung-won finds out that Eun-gyu has moved into the house next to hers. Eun-gyu now threatens to tell Jung-won’s parents that she works part time at a local mart. To keep Eun-gyu quiet, Jung-won agrees to carry his guitar for a week.

Jung-won then starts to develop feelings for Eun-gyu, who is the lead singer of a band called Do Re Mi Fa So La Si Do. Eun-gyu also starts to develop similar feelings for Jung-won. The two soon become a couple.

With an odd twist of fate, Jung-won went to the band's rehearsal room and meets a familiar person from her past, a boy named Hee-won. Their relationship makes everything more complicated. For 10 years , Jung-won and Hee-won were close friends, but because Jung-won did something that broke Hee-won's family apart they stopped talking to each other completely. And Hee-won, seeking revenge at Jun-won, allow his "friends" to beat her rendering her crippled. Jung-won is now surprised to learn that her ex-close friend is now best friends with her boyfriend.

Hee-won admits that he still likes his best friend despite what she did and he wants her back. Jun-won accepts him back into her life despite what he did and when Hee-won makes her choose between him and Eun-gyu, Jun-won makes the choice of going with Hee-won. Which leaves Eun-gyu devastated, he gives her one last chance to come back to him or else he will never be able to sing again but she breaks his heart once more and chooses Hee-won after he begged her.

A few months after it happened Jun-won hears that Eun-gyu had gotten into an accident and was diagnosed with temporary amnesia, he forgets all about her. She then spends all her time trying to get him to remember. The last scene is an re-enactment of the bands last show in which Eun-gyu gave her a last chance. The movie ends with him remembering everything and begging Jun-won not to leave him again in which she promises she never will, and with the credits showing how all of their lives improved when it all ended.